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<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
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    <title>Document</title>
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<body>

    <script>
    /* number.string.boolean.undefined.null */
    var num=10;
    var str="hello world"
    var b=true;/* boolean只有两种 true false */        
    // typeof 操作符进行判断
    console.log(typeof num);
    console.log(typeof str);
    console.log(typeof b);
    </script>
</body>
</html>